1. Field of the Invention
The present invention relates to a semiconductor storage device using a semiconductor element. The present invention particularly relates to a semiconductor storage device using an element including an oxide semiconductor.
2. Description of the Related Art
Storage devices using semiconductor elements are broadly classified into two categories: a volatile device that loses stored data when power supply stops, and a non-volatile device that retains stored data even when power is not supplied.
A typical example of a volatile storage device is DRAM (dynamic random access memory). DRAM stores data in such a manner that a transistor included in a memory cell is selected and charge is stored in a capacitor.
When data is read from DRAM, charge in a capacitor is lost on the above principle; thus, another write operation is necessary every time data is read out. Moreover, since a transistor included in a memory cell has a leakage current, charge flows into or out of a capacitor even when the transistor is not selected, whereby a data retention period is short. For that reason, another write operation (refresh operation) is necessary at predetermined intervals, and it is difficult to sufficiently reduce power consumption. Furthermore, since stored data is lost when power supply stops, an additional storage device using a magnetic material or an optical material is needed in order to hold data for a long time.
Another example of a volatile storage device is SRAM (static random access memory). SRAM retains stored data by using a circuit such as a flip-flop and thus does not need a refresh operation. This means that SRAM has an advantage over DRAM. However, cost per storage capacity is increased because of the use of a flip-flop or the like. Moreover, as in DRAM, stored data in SRAM is lost when power supply stops.
A typical example of a non-volatile storage device is flash memory. Flash memory includes a floating gate between a gate electrode and a channel region in a transistor and stores data by holding charge in the floating gate. Therefore, flash memory has advantages in that the data retention time is extremely long (almost permanent) and a refresh operation which is necessary in a volatile storage device is not needed (e.g., see Patent Document 1).
However, a gate insulating film included in a memory cell deteriorates by tunneling current generated in writing, so that the memory cell stops its function after a predetermined number of write operations. In order to reduce adverse effects of this problem, a method of equalizing the number of write operations for memory cells is employed, for example, in which case a complicated peripheral circuit is needed. Moreover, employing such a method does not solve the fundamental problem of lifetime. In other words, flash memory is not suitable for applications in which data is frequently rewritten.
In addition, flash memory needs high voltage for holding charge in the floating gate or removing the charge, and a circuit for generating high voltage is also necessary. Further, it takes a relatively long time to hold or remove charge, so that it is not easy to increase the speed of write and erase operations.